See the resources page for a website with stat info for all kart parts that you can experiment with yourself. (Including mini-turbo and invincibility stat info, which are hidden in-game.)
As of version 2.3.0 (Wave 4 of the BCP), the optimal combo for speedruns is Teddy Buggy/Rollers/Paper Glider, using Yoshi for 150cc and Yoshi or Luigi for 200cc.
Yoshi = Birdo = Daisy = Peach; Luigi = Iggy = Kamek; Teddy Buggy = Cat Cruiser; Roller = Azure Roller; Paper = Cloud = Flower = Parachute Glider
The mini-turbo stat was buffed for MK8DX compared to the original MK8, giving more boost for a longer duration overall. The mechanics of the stat itself were also changed such that it also affects the speed with which mini-turbos charge and the length and power of the boost. In addition, fire-hopping and other related tech from MK8 don't work in this game, increasing the impact of mini-turbos. This means that, while having a high base speed is still crucial, it is now much more important to temper pure speed with lots of mini-turbo.
Nintendo has added balance patches with each wave of the booster course pass, changing how weight works, adding a hidden invincibility stat, and buffing characters and vehicles they deemed underused. These buffs changed the meta from Waluigi with Biddybuggy (unchanged stats) to Yoshi/Luigi with Teddy buggy (all heavily buffed). The new meta is statistically superior to the old meta in every way.
While this build is usually the best, there are definitely situations where other combos stand out. I’ll list and address as many as I can think of:
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Beginners. Edit for 2.3.0+: The meta combo is now even more beginner-friendly! Note that Yoshi is easier to use than Luigi on 200cc due to higher handling and mini-turbo.
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Items runs. Since items can stop you or even just slow you by blocking your usual route, it is arguably important to build more acceleration, handling, and invincibility into combos for items runs. However, Yoshi/Teddy has enough acceleration and handling that it stands out as the best for even item runs. Picking a baby character, for example, would lose significantly more time due to its poor top speed than it gained due to its increased acceleration. The only exception I know of is the possibility of replacing rollers with GLA tires on 150cc, which gains +1.00 invincibility and a bit of speed at the cost of -0.50 mini-turbo.
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Some individual cups or track sets. The Yoshi/Teddy build provides a general solution to all tracks, but characteristics of some cups or track groups can call for a different approach. For example, with many high-traction parts receiving buffs throughout the BCP, the prevalence of slippery terrain in the Banana and Cherry cups means Streetle is a viable choice for those cups. Meanwhile, the Egg cup has lots of courses with anti-gravity sections, so Leaf Tires compete with Rollers here (this is a niche choice, though; Rollers are almost always better than Leaf).
Finally,
- Time trials. Speedrun.com doesn’t keep track of this aspect of the game (check Resources for a link to a site that does), but individual tracks provide a chance to meticulously optimize, which leads to a much higher pool of viable builds than full-game speedruns. If you want to know what the best of the best use, then you should check out the world record ghosts available through the in-game time trial menu and on the MK8DX Records YouTube channel.
At the end of the day, GOOD DRIVING is still the single most important factor for going fast! The game is more balanced than ever, and you can go very far without using the single optimal combo. Practice and experience trumps a Pareto optimal combo any day of the week.
Good luck on your runs!
—Pianist15
